In the 1940's the disease originally referred to as viral hepatitis was distinguished into two separate disorders termed infectious hepatitis (hepatitis A, HAV) and homologous serum hepatitis (hepatitis B, HBV). Transfusion of blood products had been demonstrated to be a common route of transmission of viral hepatitis. HBV was originally assumed to be the causative agent of post-transfusion hepatitis as the epidemiological and clinical features of the disorder did not fit those of HAV.
Soon after a radioimmunoassay for hepatitis B surface antigen (HBsAg) became available as a tool for identifying patients infected with HBV it became apparent that most patients having post-transfusion hepatitis were negative for HbsAg. Thus, hepatitis following blood transfusion that was not caused by hepatitis A or hepatitis B and was subsequently referred to as non-A, non-B hepatitis.
The causative agent of non-A, non-B hepatitis (hepatitis C virus, HCV) was discovered in 1989 via screening of cDNA expression libraries made from RNA and DNA from chimpanzees infected with serum from a patient with post-transfusion non-A, non-B hepatitis. To identify portions of the genome that encoded viral proteins, the libraries were screened with antibodies from patients who had non-A, non-B hepatitis. These investigators went on to show that the virus they identified was responsible for the vast majority of cases of non-A, non-B hepatitis.
The hepatitis C virus is one of the most prevalent causes of chronic liver disease in the United States. It accounts for about 15 percent of acute viral hepatitis, 60 to 70 percent of chronic hepatitis, and up to 50 percent of cirrhosis, end-stage liver disease, and liver cancer. Almost 4 million Americans, or 1.8 percent of the U.S. population, have antibodies to HCV (anti-HCV), indicating ongoing or previous infection with the virus. Hepatitis C causes an estimated 8,000 to 10,000 deaths annually in the United States. Hepatitis C virus (HCV) infection occurs throughout the world, and, prior to its identification, represented the major cause of transfusion-associated hepatitis. The seroprevalence of anti-HCV in blood donors from around the world has been shown to vary between 0.02% and 1.23%. HCV is also a common cause of hepatitis in individuals exposed to blood products. There have been an estimated 150,000 new cases of HCV infection each year in the United States alone during the past decade. The acute phase of HCV infection is usually associated with mild symptoms. However, evidence suggests that only 15%-20% of the infected people will clear HCV. Among the group of chronically infected people, 10-20% will progress to life-threatening conditions known as cirrhosis and another 1-5% will develop a liver cancer called hepatocellular carcinoma. Unfortunately, the entire infected population is at risk for these life-threatening conditions because no one can predict which individual will eventually progress to any of them.
HCV is a small, enveloped, single-stranded positive RNA virus in the Flaviviridae family. The genome is approximately 10,000 nucleotides and encodes a single polyprotein of about 3,000 amino acids. The polyprotein is processed by host cell and viral proteases into three major structural proteins and several non-structural proteins necessary for viral replication. Several different genotypes of HCV with slightly different genomic sequences have since been identified that correlate with differences in response to treatment with interferon alpha.
HCV replicates in infected cells in the cytoplasm, in close association with the endoplasmic reticulum. Incoming positive sense RNA is released and translation is initiated via an internal initiation mechanism. Internal initiation is directed by a cis-acting RNA element at the 5′ end of the genome; some reports have suggested that full activity of this internal ribosome entry site, or IRES, is seen with the first 700 nucleotides, which spans the 5′ untranslated region (UTR) and the first 123 amino acids of the open reading frame (ORF). All the protein products of HCV are produced by proteolytic cleavage of a large (approximately 3000 amino acid) polyprotein, carried out by one of three proteases: the host signal peptidase, the viral self-cleaving metalloproteinase, NS2, or the viral serine protease NS3/4A. The combined action of these enzymes produces the structural proteins (C, E1 and E2) and non-structural (NS2, NS3, NS4A, NS4B, NS5A, and NS5B) proteins that are required for replication and packaging of viral genomic RNA. NS5B is the viral RNA-dependent RNA polymerase (RDRP) that is responsible for the conversion of the input genomic RNA into a negative stranded copy (complimentary RNA, or cRNA; the cRNA then serves as a template for transcription by NS5B of more positive sense genomic/messenger RNA.
An effective vaccine is greatly needed, yet development is unlikely in the near future because: i) lack of an efficient cell culture system and small animal models; ii) a weak neutralizing humoral and protective cellular immune response; iii) marked genetic variability of the virus.
Several institutions and laboratories are attempting to identify and develop anti-HCV drugs. Currently the only effective therapy against HCV is alpha-interferon, which reduces the amount of virus in the liver and blood (viral load) in only a small proportion of infected patients. Alpha interferon was first approved for use in HCV treatment more than ten years ago. Alpha interferon is a host protein that is made in response to viral infections and has natural antiviral activity. These standard forms of interferon, however, are now being replaced by pegylated interferons (peginterferons). Peginterferon is alpha interferon that has been modified chemically by the addition of a large inert molecule of polyethylene glycol. At the present time, the optimal regimen appears to be a 24- or 48-week course of the combination of pegylated alpha interferon and the nucleoside ribavirin, an oral antiviral agent that has activity against a broad range of viruses. By itself, ribavirin has little effect on HCV, but adding it to interferon increases the sustained response rate by two- to three-fold. Nonetheless, response rates to the combination interferon/ribavirin therapy are moderate, in the range 50-60%, although response rates for selected genotypes of HCV (notably genotypes 2 and 3) are typically higher. Among patients who become HCV RNA negative during treatment, a significant proportion relapse when therapy is stopped.
In addition, there are often significant adverse side effects associated with each of these agents. Patients receiving interferon often present with flu-like symptoms. Pegylated interferon has been associated with bone marrow suppressive effects. Importantly, alpha interferon has multiple neuropsychiatric effects. Prolonged therapy can cause marked irritability, anxiety, personality changes, depression, and even suicide or acute psychosis. Interferon therapy has also been associated with relapse in people with a previous history of drug or alcohol abuse.
Side effects of ribavirin treatment include histamine-like side effects (itching and nasal stuffiness) and anemia due to dose related hemolysis of red cells and histamine like side effects.
Taken together, the proceeding facts indicate a significant need for effective small molecule inhibitors of hepatitis C virus replication that do not suffer from the above-mentioned drawbacks.